This invention relates to blends of aqueous thickener polymer compositions, and their method of use. In particular, one aspect of this invention relates to aqueous blends of associative thickeners, which are soluble in water over a broad pH range, with alkali soluble or alkali swellable emulsions, with or without hydrophobic modification (HASE or ASE thickeners respectively), in their low pH, insoluble, emulsion form.
Aqueous polymer systems, for example coatings containing emulsion polymer binders, typically use thickeners to obtain the desired degree of viscosity needed for the proper formulation and application of the aqueous system. One general type of thickener used in aqueous polymer systems is referred to in the art by the term “associative.” Associative thickeners are so called because the mechanism by which they thicken is believed to involve hydrophobic associations between the hydrophobic moieties in the thickener molecules and/or between the hydrophobic moieties in the thickener molecules and other hydrophobic surfaces. Associative thickeners typically are water soluble over a broad pH range of from 2 to 10, and are thus relatively pH-independent. Hydrophobically modified ethoxylated urethane thickeners (HEUR thickeners) are examples of associative thickeners with pH-independent solubility.
Another type of thickener used in aqueous polymer systems is referred to in the art as an alkali soluble or alkali swellable emulsion thickener, which thickens aqueous polymer systems effectively at pH values of 6 and greater because they are typically water insoluble at pH values of less than 6 and water soluble at pH values of greater than 6. The alkali soluble or alkali swellable emulsion thickeners are typically provided in the emulsion form at pH values below 5. At this low pH, the viscosity of the product is acceptable and product solids by weight can typically be in the range of 30% to 50%. Alkali soluble or alkali swellable emulsion thickeners that contain no hydrophobic groups and thicken by a non-associative mechanism upon neutralization with base are described in the art as ASE thickeners. Alkali soluble or alkali swellable emulsion thickeners that are hydrophobically modified and thicken at least partially by an associative mechanism upon neutralization with base are described in the art as HASE thickeners. As used herein, however, the terms ASE and HASE are not restricted to just the emulsion form of these polymers.
Thickeners are typically sold as pourable aqueous liquids. For ease of use, it is desirable for the viscosity (Brookfield at 6 rpm) of such thickener products to be less than 15,000 centipoise (cps), or even less than 5,000 cps, so that the product will readily drain from its storage container, and be readily incorporated into the aqueous system to which it is added. The viscosity of the aqueous thickener product can be decreased by reducing the active solids concentration, but this has the drawback of limiting formulation latitude in terms of weight solids of the aqueous system to be thickened by the product.
To provide a balance of properties that cannot be provided by a single thickener, it is desirable to be able to blend two or more different thickeners into a single product. For instance, U.S. Patent Application Publication No. US 2007/0161745A1 discloses blending a multiplicity of different HEUR thickeners to provide a unique balance of high shear rate and low shear rate viscosity enhancements.
U.S. Pat. No. 7,125,919 discloses blends of thickeners and includes the combination of HASE thickeners with HEUR thickeners. However, the embodiment also requires that the composition comprise at least one macromolecular organic compound having a hydrophobic cavity, such as a cyclodextrin compound. Unfortunately, the cyclodextrin component adds significant cost to the blend composition.
It would be useful to supply aqueous blends of pH-independent soluble associative thickeners with ASE and/or HASE thickeners at a viscosity that can be handled and poured readily without incurring the additional cost of the cyclodextrin compound. Thickeners with pH-independent solubility commonly provide much better flow and leveling in coatings than do ASE or HASE thickeners. It would be useful to improve the flow and leveling performance of ASE or HASE thickener products by providing them as a blend with a pH-independent soluble associative thickener. However, to date it has not been possible to blend pH-independent soluble associative thickeners with ASE or HASE thickeners in their low pH emulsion form at practical weight solids because the viscosity of the aqueous blend is exceedingly high, essentially existing as a gel. Thus, there remains a need in the art for inexpensive pourable blends of HASE or ASE and pH-independent soluble associative thickeners with both low viscosity and the highest active thickener solids possible.